For EACH Coaster (one fat quarter is enough to make all four Top Pieces for a set of four)
Coasters finish to about 4" square.
Cut one 4.5" square for the Bottom (coordinate color with the Top Pieces) Cut four 4.5" squares for the Top (I cut all the Top Pieces from the same fabric, but you could use different fabrics) Cut two 3.75" squares of cotton batting ( I thought they needed more padding)
Press the four Top Pieces into triangles, right sides out. Lay Bottom right side up. Lay the Top Pieces on top the Bottom, with the folds toward the center of the coaster. Put two on opposite corners, then lay the last two on the two remaining corners. Sew all around, using about a 1/4" seam. Trim off the corners Fold one Top Piece to the back, then slip both batting squares under that piece. Fold the next Top Piece to the back, tucking the batting into the corners nice and even. There should now be two Top Pieces covering the back and two Top Pieces still on the other side. Fold the last two Top Pieces to the back. Press and then pin the Top Pieces to hold them in place. Sew all around the outside, sewing in about 1/2" from the edges. Next sew two diagonal lines to hold down the Top Pieces.